A conventional helium cooling apparatus of this type is arranged, as shown in FIG. 1. The helium cooling apparatus 1 comprises a liquid helium container 3 which stores liquid helium 6 at a predetermined liquid level, and a condensation chamber 5 which incorporates a condensation heat exchanger 4. The container 3 is arranged in a cryostat 2. An object 7 to be cooled (e.g., a superconducting magnet) is immersed in the liquid helium 6 in the container 3. The liquid helium container 3 has a port 9, and a pipe 10 open to the external atmosphere is connected to the port 9. The liquid helium container 3 communicates with the condensation chamber 5 through a transfer tube 11. The transfer tube 11 is inserted into the pipe 10 and the port 9. The condensation heat exchanger 4 is connected to a refrigerator 8 for supplying a refrigerant to the heat exchanger 4.
The liquid helium 6 in the liquid helium container 3 is gradually evaporated by a heat energy transferred from the external atmosphere to the container 3. The evaporated gas helium is supplied to the condensation chamber 5 through the transfer tube 11. The temperature of a heat conduction surface of the condensation heat exchanger 4 is set at 4.2 K. The gas helium is condensed again into liquid helium by the condensation heat exchanger 4. The liquid helium descends in the transfer tube 11 by gravity and returns to the container 3. The amount of liquid helium 6 in the container 3 is kept constant, and the object 7 can be satisfactorily cooled.
When the heat energy transferred to the liquid helium container 3 is increased, the amount of the gas helium evaporated in the liquid helium container 3 is increased. Therefore, the flow rate of the gas helium ascending through the transfer tube 11 is increased, and the liquid helium descending through the transfer tube 11 is forced to ascend together with the gas helium. More specifically, the inside of the transfer tube 11 is blocked by the gas helium ascending through the transfer tube 11, so that the liquid helium cannot descend through the transfer tube 11. (This phenomenon is called a flooding phenomenon.) As a result, the liquid helium 6 in the liquid helium container 3 is continuously evaporated by the heat energy transferred from the external atmospher to the container 3, and the amount of the liquid helium 6 in the container 3 is decreased. As a result, it is difficult to cool the object 7.
The flooding phenomenon is determined by the inner diameter of the transfer tube 11, the flow rate of the liquid helium descending through the transfer tube 11, and the flow rate of the gas helium ascending through the transfer tube 11. The flow rates of the liquid helium and the gas helium are determined by the heat energy transferred from the external atmosphere to the liquid helium container 3.
The present inventors made several tests, using the conventional helium cooling apparatus, to obtain a relationship between the level of the liquid helium in the container 3 and the lapse of time and relationship between the level of the liquid helium in the chamber 5 and the lapse of time. In these tests, the transfer tube 11 of the helium cooling apparatus has an inner diameter of 5 mm, and the heat energies of 0.5 W and 0.7 W were transferred from the external atmosphere to the liquid helium container 3.
As is apparent from FIG. 2, when 0.5 W heat energy is transferred to the liquid helium container 3, the liquid level in the container 3 is kept constant independently of time lapse. However, when the heat energy of 0.7 W is transferred to the liquid helium container 3, the flooding phenomenon occurs. In other words, most of the liquid the helium in helium container 3 is evaporated by the heat energy and is converted into the gas helium, which is then fed to the condensation chamber 5 through the transfer tube 11. Thus, the inside of the transfer tube 11 is blocked by the gas helium ascending through the transfer tube 11, so that the liquid helium in the condensation chamber 5 cannot descend through the transfer tube 11. Accordingly, when the heat energy of 0.7 W is transferred to the liquid helium container 3, the liquid level in the container 3 is lowered, while the liquid level in the chamber 5 is raised. Thus, if the inner diameter of the transfer tube 11 is 5 mm, the helium cooling apparatus cannot have a cooling capacity of 0.7 W or more. For example, even if the refrigerator 8 has a refrigeration capacity of 4 to 5 W and the condensation heat exchanger 4 has a condensation capacity of 4 to 5 W, the energy subjected to actual condensation is 0.7 W or less. In order to prevent the flooding phenomenon and to maximize the condensation capacity of the helium cooling apparatus, the inner diameter of the transfer tube 11 must be relatively large.
When the inner diameter of the transfer tube 11 is relatively large, the sizes of the port 9 and the pipe 10, both of which receive the transfer tube 11, must be increased. As a result, the heat energy transferred from the external atmosphere to the helium container 3 through the port 9 and the pipe 10 is increased. In other words, when the inner diameter of the transfer tube 11 is increased, the flooding phenomenon in the transfer tube 11 can be prevented, and a satisfactory cooling capacity of the helium cooling apparatus can be obtained. However, the heat energy transferred to the helium container 3 is undesirably increased. For this reason, the inner diameter of the transfer tube 11 must be decreased. In this case, however, a flooding phenomenon occurs, and the satisfactory cooling capacity of the helium cooling apparatus cannot be obtained.
It is, therefore, difficult to obtain a satisfactory cooling capacity of the helium cooling apparatus while the inner diameter of the transfer tube 11 is kept small and the flooding phenomenon is prevented. In other words, it is difficult to obtain a satisfactory cooling capacity of the helium cooling apparatus while the apparatus is kept compact.